Two years ago, players were complaining that the dungeons of World of Warcraft's Cataclysm expansion were not fun because they were too difficult. The Blizzard response - that content should possess non-zero challenge - was as accurate as it was irrelevant. Customers were dissatisfied with the level of fun they were having with the results of the design, not the quality of the design itself.
Today, Blizzard argues that various non-raid activities - such as daily quests and running the looking-for-raid difficulty in pick-up-groups - are optional for raiding because only the very top difficulty setting is balanced so tightly as to assume that players have the best gear available. I like to call this the "pants optional" argument - no MMO I am aware of has a mandatory requirement that characters wear pants, but very few players opt to go pantless. The choice technically exists, but is largely uninteresting, as there is almost always (*) no benefit to going without pants and the player would then be obligated to upgrade the rest of their gear to off-set the stats from the missing leggings. More to the point, every bit by which you exceed the theoretical minimum requirement gives the player - and the group of 9-24 friends they are raiding with - that much more margin for error to help secure victory.
We could sit here and argue the academic/semantic merits all day, but this misses the point for the same reason Blizzard's 2011 defense of the game's difficulty missed the point. If paying customers feel like they are obligated to do something that they do not believe is fun, it does not matter if the customer is theoretically incorrect. Lecturing the customer on why they are incorrect, not as good at playing the game as people who are beating the content with the minimum gear, and need to find new friends with lower expectations - however accurate all of these statements may be - is not a good business strategy.
The structural issues with Cataclysm as an expansion probably would not have gone away had the game's initial cadre of heroic dungeons launched with lower difficulty and shorter completion times. Even so, it was an inauspicious start to what turned into the game's least successful era to date. If Blizzard continues to build a game whose core endgame mechanic is upgrading character performance through acquisition of better gear, and continues to require non-raid content for access to upgrades that raid players want, Pandaria may not be off to any better of a start.
(*) - There have been several eras of WoW in which certain tanking classes were obligated to intentionally lower their mitigation when attempting content that was significantly below their gear level, because their classes were dependent on taking sufficient quantities of damage in order to generate resources. Several players I knew would remove their characters' pants in this scenario, because it was the quickest and most humorous way to accomplish this.